1. Field of Invention
The present invention relates to a technique for starting a brushless DC (BLDC) motor. More particularly, the present invention relates to a technique for starting a sensorless BLDC motor.
2. Description of Related Art
Brushless DC motor (BLDCM) with rare-earth-element permanent magnets has been implemented in more and more wide range in recent years due to its outstanding advantages such as high efficiency, high power density, good controllability, and so on. A BLDC motor requires an inverter and a position detector to perform commutation. Conventionally, three Hall sensors are used as a position sensor for the BLDCM. However, the position of the Hall elements must be very precisely fixed, and the sensors themselves cause increase of the cost and reliability penalty. Many position sensorless BLDCM drivers, which detect the rotor position and produce the commutation signal from the back electromotive forces (BEMF) induced in the stator, have been proposed.
However, all of such sensorless motors face a startup problem: because the induced BEMF are not present when the motor is at rest, and the starting position of the rotor is unknown. For these reasons, several startup procedures have been developed in order to overcome this technical difficulty. Awell-known method called 3-stage startup procedure consists in the following steps. First, a predefined initial phase is excited for a preset time, in order to push the rotor to a specific, known position. Such a procedure is called as orientation. Thereafter, a series of synchronous signals should be generated sequentially to accelerate the rotor in an open-circuit mode by increasing the frequency of such synchronous signals. This stage is called as separately controlled accelerating. When the motor reaches a speed at which rotor position signals become available and reliable, it comes to the stage of switching to self-controlled operation. The drawbacks of this startup procedure are that the rotor may not properly follow the excitation sequence and that a relatively long time is required.
Due to difficulty to decide the initial rotor position and the purpose of step-losing protection, the current at the startup has to be greater than the load torque plus the friction torque and the motor inertia load. Usually, twice rated current (or less) is excited to the motor to start up fast. So the frequently startup of the drivers causes plenty of copper loss and over heat at last, if the startup time is long and startup current is large always. Thus, there is a need for a fast, startup procedure for sensorless BLDCM.
The operation principle for a three-phase motor is briefly described as follows. The conventional three-phase motor is shown in FIG. 1. In FIG. 1, the three-phase motor is under a star configuration, i.e., having six different excitable phases and six equilibrium points in a 360 electrical degree. In the following description, each excitation phase is indicated with two capital letters, in whicha first letter (A, B, C) designates the winding through which the current conventionally flows from a supply terminal and toward the star node (N). The second capital letter with a “_” sign designates the winding through which the current comes from the star node (N) and flows toward another supply terminal.
FIG. 2 shows the torque curves of the motor in relation to different phase of excitation, as well as the corresponding curves of the three BEMFs. In FIG. 2, for each excited phase, for example AB_, the rotor tends to dispose itself in coincidence with the stable equilibrium points t180. Therefore, for a total of six phases there are six equilibrium points in 360 electrical degrees and there are many points of mechanically unstable equilibrium which are shifted by 180 electrical degrees from the respective stable equilibrium point. Moreover, it is evident that by exciting the phase AB_while the rotor is at rest, either a forward motion, or a backward motion, or no motion at all may occur, as shown in Table 1. The sign “-” of the torque means that the torque will startup a motor in a backward direction of rotation. The sign of the BEMF of the floating phase C is also shown, when the rotor runs in forward motion as shown in FIG. 2.
TABLE 1Rotor positionTorqueBEMF of phase CRotation direction0–t1++Forwardt1–t180+−Forwardt180–t4−−BackwardT4–0−+BackwardT1800Rest
A method with better efficiency of startup procedure for a sencorless BLDC motor, based on the foregoing basics properties of the three-phase motor is till desired.